GNSS Number of Satellites RNAV

All versions of GNSSGNSS —Global Navigation Satellite System use the intersection of spheres based on your range from a satellite to find your position.

  • Two spheres intersect in a circle, which is of no use.
  • Three spheres intersect at two points. While this is still ambiguous to a point, if you make the assumption you are on the surface of the earth, you can usually eliminate one of the positions to give yourself a 2D fix.
  • Four satellites lifts the ambiguity and gives you a 3D or 4D fix (4D = latitude, longitude, altitude and time.)
  • Five satellites are used by RAIMRAIM —Receiver Autonomous Integrity Monitoring to check this calculated position. It will tell you if something is wrong, but it does not know what - this could be the original calculated position or the "advice" given by the 5th checking satellite.
  • Six satellites are used by FDEFDE —Fault Detection and Exclusion. This will not only tell you something is wrong, and which satellite is providing erratic information and enable you to exclude that satellite signal when calculating your position.

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With the NAVSTAR GPSGPS —Global Positioning System (USA) system, how many satellites are required for a 2D fix?
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